ABSORPTION OF FAT. 261 



that the proteids are absorbed, and that the capillary blood-vessels 

 of the villi are the efficient agents in this process. Some recent 

 experimenters, Asher and Barbera, claim that some proteid is 

 absorbed by the lacteals, but Mendel conducted an investigation 

 as a result of which he concludes that " under ordinary circum- 

 stances by far the greater share in the process must still be dele- 

 gated to the capillaries of the villi." 



Although proteids are mainly absorbed as proteoses and pep- 

 tones, yet during this act they lose their identity. In other words, 

 while passing through the epithelial cells which cover the villi they 

 are so changed that neither proteose nor peptone can be found in 

 the blood, and if these substances are directly injected into the blood 

 they are eliminated by the kidneys and are found in the urine. 

 Indeed, if the amount is sufficient they act as poisons, causing 

 insensibility, lowering blood-pressure, diminishing or destroying 

 the coagulability of the blood, and producing death. Although 

 the power to convert proteoses and peptones into a form which 

 can be assimilated is claimed for the leukocytes present in the 

 intestinal wall, the evidence seems conclusive that this claim is 

 without substantial foundation, and that it is to the columnar 

 epithelial cells covering the villi that the change is to be attrib- 

 uted. Up to the present time the form of proteid into which 

 these substances are changed has not been determined, though it 

 is doubtless a coagulable proteid, and probably serum-albumin 

 or globulin. 



Absorption of Vegetable and Animal Proteids. The 

 products of vegetable proteolysis are not so completely absorbed 

 as are those of animal origin. It is stated by Moore that this 

 is in part due to their envelopes of indigestible cellulose, in part 

 to their shorter stay in the intestine because of their action in 

 causing increased peristalsis, and in part to their less digestible 

 character. He further states that the proteids of some legumin- 

 ous plants and cereals are absorbed nearly as perfectly as those of 

 animal origin, while in most others (potato, lentil) it is much less 

 complete (22 to 48 per cent. less). The percentage of the nitrogen 

 of meat or egg appearing again in the feces in man amounts to 

 but 2.5 to 2.8 per cent. ; that of milk, to but 6 to 12 per cent. 



Absorption of Fat. There is no dispute as to the lacteals 

 being the channels through which the fat reaches the blood-vascular 

 circulation by way of the thoracic duct ; the presence qf fat in 

 the vessels has been observed too often to admit of any doubt 

 on this point. Indeed, it is the milky appearance given by the 

 fat-particles to the contents of these vessels which has given to 

 them the name of lacteals (Fig. 120), but as to the manner and 

 form in which fat passes into the villus to reach the lacteals two 

 theories are held : (1) the emulsion theory and (2) the solution 

 theory, or, more correctly, solution theories. 



Emulsion Theory. This theory is the older, and explains the 



